Corresponding field devices, especially Coriolis mass flow meters or Coriolis mass flow/densimeters, especially because of their versatility, are widely applied in process and/or automation technology and are manufactured by the company, Endress+Hauser in great diversity and sold, for example, under the mark PROMASS. They are most often integrated as in-line measuring devices in compact construction into the relevant pipeline and have a housing module, which is mechanically coupled with the pipeline via an inlet end and an outlet end. The measuring transducers comprise, furthermore, a sensor module having at least one measuring tube held oscillatably in the housing module, communicating with the pipeline, and executing, at least at times, oscillations, especially bending oscillations, about a static resting position. Furthermore, a measuring device of the field of the invention includes at least one electromechanical, especially electro-dynamic, exciter mechanism acting on the at least one measuring tube for producing and/or maintaining mechanical oscillations of the at least one measuring tube, and at least one vibration sensor arrangement reacting to oscillations of the at least one measuring tube for detecting the oscillations of the at least one measuring tube and for producing at least one oscillatory measurement signal representing oscillations.
The underpinning measuring principles are known from a large number of publications and are described at length and in detail, for example, in U.S. Pat. Nos. 4,793,191, 4,823,614, 4,831,885, 5,602,345, US-A 2007/0151368, US-A 2010/0050783, and published International Applications, WO-A 96/08697, WO-A 2009/120222 and WO-A 2009/120223.
In the course of time, many different embodiments have been proposed and used for the field devices of the field of the invention. Thus, embodiments have been described with 1, 2, 4, or 8 measuring tubes connected in parallel, embodiments, in the case of which the at least one measuring tube is essentially straight, and embodiments, in the case of which the measuring tubes are at least sectionally bent, especially essentially U-, V-, or trapezoidally shaped. The at least one measuring tube is most often manufactured of a metal, especially titanium, zirconium, tantalum or stainless steel, and arranged at least partially within the housing module. For integration of the field device in an existing pipeline, the field device is provided on its in- and outlet ends, in each case, with a process connection, especially a flange.
For field devices with at least two measuring tubes, so-called distributor pieces are provided, one in the region of the inlet end and one in the region of the outlet end process connection. These two distributor pieces are connected mechanically with the housing module, and serve for dividing the flowable medium among the relevant number of measuring tubes, and for the inlet end and the outlet end mechanical connection of the at least two measuring tubes with one another. Additionally, frequently used for field devices with at least two measuring tubes are so-called coupling elements, by means of which the at least two measuring tubes are mechanically coupled at the inlet end and the outlet end.
The coupling elements serve as oscillation nodes.
In contrast, the connecting to the pipeline in the case of field devices with only one measuring tube occurs by means of essentially straight connecting tube pieces, one at the inlet end and one at the outlet end. Furthermore, field devices with only one measuring tube include at least one counteroscillator, which can be embodied as one-piece or a plurality of parts, and which is coupled to the measuring tube for forming inlet end and outlet end coupling zones. In operation of the field device, the counteroscillator either rests, or it is excited to execute oscillations with equal frequency, however, opposite phase, relative to the measuring tube. For example, the counteroscillator can be embodied as an essentially tubular, hollow cylinder, in such a manner that the measuring tube is at least partially jacketed by the counteroscillator.
In operation, the at least one measuring tube is excited to mechanical oscillations in the so-called drive- or wanted mode with the so-called wanted frequency, which is usually a frequency corresponding to an oscillatory mode of the at least one measuring tube, such that the at least one measuring tube executes resonant oscillations. The mechanical oscillations in the wanted mode are, in the most frequently arising case, in which these oscillations correspond to the resonant frequency in the fundamental mode, especially in the case of a Coriolis mass flow- and/or densimeter, as a rule, at least partially embodied as lateral, bending oscillations. In such case, there forms usually in the region of the two ends of the at least one measuring tube, in each case, an oscillation node and in the intermediately lying region exactly one oscillatory antinode. However, also known are applications, in which the torsion mode is excited.
When the medium flows through the at least one measuring tube, there are induced within the measuring tube reaction forces, which lead, in addition to the oscillations in the wanted mode, to equal frequency oscillations in the so-called Coriolis mode, which are superimposed on the oscillations in the wanted mode and, thus, are included in the oscillatory measurement signal. Depending on the type of the induced, and detected, reaction force, various process variables can be registered. For example, the mass flow corresponds to the Coriolis force, the density of the medium to the inertial forces and the viscosity to the friction forces.
The accuracy of measurement, and, partially associated therewith, also the possible application domain, of a field device of the field of the invention of the above described type depend, in such case, on many different factors.
On the one hand, external disturbing influences can negatively influence the accuracy of measurement. These include, for example, vibrations of the pipeline and/or of the housing module, which can couple into the oscillations of the at least one measuring tube. However, also disturbing vibrations from pressure fluctuations of the flowing medium can be problematic, or even different temperature loadings of the different components. Moreover, the accuracy of measurement depends also on how the at least one measuring tube is secured in the housing module. Movements of the end regions of the at least one measuring tube in the housing module can lead, for example, to clamping forces, which act on the process connections and, in given cases, also on the distributor pieces and can lead to deformations of the housing module. It is thus desirable effectively to minimize such and also other, disturbing influences.
For example in this regard, different structural measures relative to the mechanical construction and the connections of the individual components of the particular measuring transducer can be applied. The connections between the at least one measuring tube, the process connections and, in given cases, distributor pieces, as well as the housing module should be as stable and stiff as possible. Moreover, the opportunity for transmission of oscillations from the housing module to the at least one measuring tube should be reduced. It is, however, a fact that many of these measures are accompanied by a marked weight gain, which, in turn, is undesirable, especially in the case of field devices of larger nominal diameter.
On the other hand, the accuracy of measurement depends also on the oscillation characteristics of the at least one measuring tube. Here, special attention should be paid to the frequency spectrum, thus the position of the frequencies of the oscillation modes of the at least one measuring tube. This depends, among other things, on the size, geometry, stiffness, mass distribution and material, of which it is manufactured, as well as, in given cases, on the instantaneous density, viscosity and/or temperature of the respective medium.
An important aspect with reference to an as disturbance free as possible measuring is the relative positions of the frequencies of the oscillation spectra of the at least one measuring tube and the housing module. The oscillation modes of the two components should namely not lie at the same frequencies. Since the resonance frequencies depend basically both on the mass distribution as well as also on the stiffness of the respective components, the positions of the individual oscillation modes can be influenced by variation of these parameters. These measures have, however, structural limits, since certain geometries considered advantageous from the physical standpoint, e.g. certain tube shapes, or radii of curvature, of the at least one measuring tube, are not or only difficultly and/or very complicatedly implementable with established manufacturing processes.
Furthermore, it is desirable for a high accuracy of measurement that the stiffness of the at least one measuring tube be matched to the particular application. For example, the amplitude of the oscillatory measurement signal depends on the stiffness of the measuring tube. Now, it is, however, a fact that a change of the mass distribution changes not only the stiffness but, instead, also the position of the oscillation modes within the oscillation spectrum of the at least one measuring tube. However, it would often be advantageous to vary either only the stiffness at equal remaining frequency spectrum or only the frequency spectrum at equal remaining stiffness. Such requirements can, however, currently only be met with extreme difficulty and/or complications.